Passive reflector



May 10, 1960 H. P. COLEMAN 2,936

I PASSIVE REFLECTOR I Filed July 2, 1957 2 Sheets-Sheet 1 INVENTOR H.PARIS COLEMAN I '7 ATTORNEYJ May 10, 1960 H. P. COLEMAN 2,936,453

PASSIVE REFLECTOR Filed July 1957 2 Sheets-Sheet 2 INVENI'OR H. PARISCOLEMAN ATTORNEY5 Pat I the.Na'vy' V I V i P qii i 11951; sfial 669,689

.6Claiins. (03343 -915) i I I transmissive. cede 1952 see. 266)hereinmay be manufactured used by or for theG overnment of the UnitedStates of Amerrcafor governmental purposes without the paymerit of 'anyr oyalties thereon or therefor.

This invention relates in general to a lens antenna and m particularlyto a passive microwave reflecting lens utilizing an artificialdielectrical medium.

'li lectrical signal reflectors :constructed of concentric rings ofdielectricmaterial with a varying dielectricrcon stantaare well known inthe art as adaptations of the lens principle pointed outin R. L.Lunebergs Mathematical. 'Iiheory o f opticsi, pages 208-213. Suchstructure af fogdg;a .;reflectorqwithexcellentfocusing abilities over awide solid angle of scan and results in'max'imum antenna gain ,Thisapparatus has been limited in its applicationbecause f the'relativelylarge bulk and Weight oli;th,e:lens. ,Recentlypadvances have been madein de-.

creasing: thei.w eight of. such antenna designs by use of artificial3dielectric, mediums but in many applications especially where size andweightare a critical consideration, the present.,lens apparatus isgenerally unsuitable.

. There areothertypesof antennas that do providelightweightzandcompactness, for example, a corner reflector is..-a,,device..which. r'eflectsa' fairly well focused signalwlienjthetdirectional alignment is'accurate. However, theficornerreflector. cannot even closely approach the uniform ;-response of areflecting'lens over a wide angle since it does not reflect eflicientlyexcept at several rather limited angles, there being a markeddeterioration of the response of a corner reflector at angulardisplacements of more than several degrees" from the optimum responseangles/ I 'There'fofeg'it is an object of this invention to provide a-reflector antenna having the characteristics and advantag's of a soliddielectric lens without the inherent accompanyifig disadvanta e of greatbulk and w i ht.

Another object is to provide such a lens in a structure capable ofexpansion from a compact initial volume to emerge hilly-expanded volume.I

*;no tlie"r object is to provide a method of creating an artificialdielectric medium of varying dielectric-constant er expanding such alensfrom an initial volume to a liii'ge 'fully-expaiidedvolume, suchexpansion to be unifoir'n "and accomplished Without the possibility ofenringing the lens components.

.Fig. 4 is an illustration of the lens when it is partially :expandedInbrief, the'lensdescribed herein comprises a'plurality of extensiblefilaments of dielectric material withv a plurality of conductingobstacles spaced in or on said extensible filaments at. predeterminedlongitudinal intervals,-sa'id extensible filaments positionedlongitudinally between two end plates in a predetermined variation ofdensity and radius configuration to form a dielectric me-v dium ofsuitable non-uniform dielectric constant. The aforementioned structureis enclosed by an envelope having a' plurality of bellows-likepleats inplanes parale lel toj the? two-end plates, said bellows-like pleatsfacili tating efxpansionalong an axis normal to the two end plates andparallelto the extensible filaments. On one portion of the envelope is areflector shield covering less than of the circumference, made suitablefor, reflecting collimated rays for electromagnetic energy arriving atthe lens.- surface of the reflector shield. Such a lens will provide amicrowave energy reflecting assembly with excellent focusingcharacteristics through a large solid angle of scan.

Referring now to Fig. 1, the structure of the. cylindrical reflectinglens contains a plurality of extensible filaments 11 which are adielectric made of plastic, paper or other 7 suitable material;Filaments 11 support a plurality of conducting obstacles 11(a) such asimbedded" metal pins, a metallic coating, or the like, which, arepositioned lengthwise on' 01 therein. The extensible filaments -11 areattached longitudinally between the circular end plates 12 which may beof any lightweight non-conductor or'diele'c'tr'ic such as plastic,cardboard or other suitable material into which the extensible filaments11 may preferably be molded. An envelope 13 of plastic,,paper or anyother .niaterial of suitable weight and strength is:

spacers 15' may be of any lightweight substance such as plastic, paperor other suitable material and the. apertures 15(a) through which thefilaments 11 are threaded perferably correspond to the filament spacingon the circula'r end plates 12. The spacers 15 also contain vents 15(b)to allow an expansion medium such as air, carbon dioxide, or othersuitable material, to flow throughout the envelope 13. To retain theprescribed pattern of location of the extensible filaments 11, thespacers 15 are situated longitudinally throughout the envelope 13. Theextensible filaments 11 are threaded through apers tures 15(a) in thespacers 15, thus maintaining proper location of the extensible filaments11 in the expanded position and further serve the purpose of preventingsnarling of the extensible filaments 11 when the envelope 13 is in theu'n'expanded position or being expanded. The spacers 15 also aid ingiving the lens rigidity when such lens is fully expanded. The metallicreflector shield 16 may be achieved by a metallic coating or point onthe surface of the envelope 13 and may be equal to or less than 180 incircumferential surface.

It is well known in the art that the normal energy propagationalcharacteristic of a medium such as'air can be altered by the placementof conductors therein. By.

selecting the size and spacings of such conductors, the

2,936,455 j Patented May n aeo Such rays are focused on the innerdielectric constant may be eflectively altered substantially uniformlyin a selected manner. To achieve suitable energy propagationalcharacteristics in this invention the conductors 11(a) are spaced uponthe extensible filaments 11 which are attached to the end plates 12 inthe proper variation of density and radius configuration to simulate thealteration'of dielectric constant required fora selected end result.

When the lens is fully expanded, the extensible filaments 11 form acylindrical dielectric medium of varying dielectric constant Itsatisfying the Luneberg formula center of the lens to the radial pointwhere the dielectric constant is being determined. Then, as shown inFig. 2,

collimated rays of electromagnetic energy entering the lens will befocused at a point S diametrically opposite the entrance of the ray wpassing through the center of the lens. The reflector shield 16 shouldpreferably cover somewhat less than 180 in circumferential surface toprevent undue shadowing of the collimated rays such as y and z, farthestaway from the radius vector ray w, and still give the lens its greatestsolid angle of reflecting coverage. As taught by the art in Modificationof the Luneberg Lens to Perform as High-Gain Wide Angle Reflector, B.Gorr and F. S. Holt, Air Force Cambridge Research Center, InternalMemorandum, February 15, 1956, the shadow losses are negligible for a 90reflector shield and increase as the reflector shield is enlarged inangular coverage. At 140 there is about 3 db loss in antenna gain andthe shadow losses increase so rapidly thereafter as to make a largerangle reflector shield impractical. Therefore, 140 is recommended as ahigher limit of the solid angle of scan and circumferential surface ofreflector shield 16. Thus nearly all of the rays striking the reflectorshield 16 at a point focus are reflected back through the lens to form acollimated beam of exceptional focus maximizing antenna gain.

The lens provides exceptional focus throughout a wide angular sectorthereby eliminating the necessity of close directional alignment as isnecessary with other apparatus. The lens further provides a radar markeror target for navigation or rescue signalling that requires no visualperception to precisely align such apparatus as a corner reflector sincerotation of the lens through little more than half revolution will givea 360 coverage.

With reference to Figs. 3 and 4 it is apparent that the lens is madecapable of expansion from a compact size Fig. 3 through partialexpansion Fig. 4 to a large fully expanded volume of Fig. 1 by theelongation of the accordion-pleats of envelope 13. The support rings 14typically used to form the accordion-pleats give the structure stabilityand control the uniform cylindrical shape while the envelope 13 is beingexpanded. The expansion of the antenna structure may be accomplished byapplying a pressure differential within the envelope 13 either bychemical or physical means typically through a flexible tubular member17 which can be closed off when desired. The lightweight of thestructure makes normal respiratory pressure of the human body adequatefor expansion. As the lens expands, the pressure difierential passesthrough the vents 15(b) provided in the spacers 15 to all sections ofthe envelope 13, expanding each section and pleat smoothly and uniformlyas shown in Fig. 4. The spacers 15 and support rings 14 furnish rigidityto'the expanded envelope 13 and the spacers 15 also aid in maintainingthe proper extensible filament 11 location. Further, by containing theextensible filaments 11 at several intervals throughout the envelope 13,the spacers 15 shorten the free lengths of the extensible filaments 11in the unexpanded position thereby decreasing the 'possibility of theirsnarling or' entangling during collapsed storage or during expansion ofthe lens.

Obviously, many modifications and variations of the present inventionare possible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A radiant energy reflector comprising, first and second end members,a plurality of conducting obstacles, extensible means for spacing 'saidplurality of conducting obstacles at predetermined locations between thefirst and second end members, a longitudinally extensible enclosurecooperative with said first and second end members for containing saidplurality of conducting obstacles, said enclosure being ofnon-conducting material, means for establishing a fluid pressure withinsaid enclosure, and means for reflecting radiant energy passing withinsaid enclosure.

2. A radiant energy reflector comprising, first and second end members,a plurality of conducting obstacles, extensible means for spacing saidplurality of conducting obstacles at predetermined locations between thefirst and second end members, spacer means for retaining said extensiblemeans between said first and second end members and substantiallyperpendicular thereto, each of said spacers containing a plurality ofapertures for maintaining selected spacing between adjacent extensiblemeans and expansion pressure vent-means for restricting the passage offluid through the spacers, a longitudinally extensible enclosurecooperative with said first and second end members for containing saidplurality of conducting obstacles, and means for establishing a fluidpressure within said enclosure.

3. In a microwave reflector lens, a plurality of conducting obstacles, aplurality of extensible dielectric filaments, means for spacing saidplurality of conducting obstacles at predetermined intervals along theplurality of extensible dielectric filaments, filament spacer means,-each of said filament spacer means having vent meansand a plurality ofapertures, each of the plurality of extensible dielectric filamentsbeing threaded through a respective one of the plurality of apertures ineach of said filament spacer means such that the plurality of extensibledielectric filaments are parallel to one another and in the form of acylindrical dielectric medium having a dielec-- tric constant whosemagnitude varies along a radius through the center of the cylindricaldielectric medium,.

two end plates, the plurality of extensible dielectric filamentsconnected between the two end plates, means in-.

cluding bellows-like pleats cooperating with the two end plates forenclosing the space between the two end plates, said bellows-like pleatsformed in planes parallel to the two end plates, means for reflectingradiant energy, said last mentioned means positioned between the two endplates such that the point of focus of any beam of rayswithin the solidangle of scan will fall on the inner surface of said last mentionedmeans.

4. A radiant energy reflector comprising first and second end members, aplurality of conducting obstacles,

extensible means for spacing said conducting obstacles:

between the first and second end members, spacer means for positioningsaid extensible means between the first and second end members andsubstantially perpendicular thereto, each of said spacer meanscontaining a plurality of apertures for maintaining said extensiblemeans in predetermined relation in the planes of said spacer means,

each of said spacer means containing vent means for per-- forpositioning said extensible means between the first and second endmembers and substantially perpendicular thereto, each of said spacermeans containing a plurality of apertures for maintaining saidextensiblemeans in predetermined relation in the planes of said spacer means, eachof said spacer means containing vent means for permitting passage offluid therethrough, an extensible enclosure cooperative with said firstand second end members, means for establishing a fluid pressure withinsaid enclosure, and reflecting means associated with said enclosure forreflecting radiant energy, said reflecting means positioned between thefirst and second end members such that the. point of focus of any beamof rays within the solid angle of scan will fall on the inner surface ofsaid reflecting means.

6. A radiant energy reflector comprising first and secend end members, aplurality of conducting obstacles, extensible means for spacing saidconducting obstacles between the first and second end members, spacermeans forpos-itioning said extensible means between the first vthreadedthrough a respective one of said plurality of apertures in each of saidspacer means such that said extensible means are parallel to oneanother'and in the form of a cylindrical dielectric medium having adielec tric constant whose magnitude varies along a radius through thecenter of the cylindrical dielectricv medium,

reflecting means tor reflecting radiant energy, said refleeting meanspositioned-between the first and second end members such that the pointof focus of the beam,

of rays within the solid angle of scan will fall on the inner surface ofthe said reflecting means, and means cooperating with said first andsecond end members for maintaining said extensible means in an extendedcon-.

OTHER REFERENCES Designing Microwave Dielectric Lens, K. S. Kelleher etaL, vol. 29, June 1956, pp. 138-142.

